The subject invention relates to downhole motors and more particularly, to apparatus for controlling the flow of drilling fluid when inserting and extracting a drill pipe from a hole.
In most drilling operations, drilling fluid or "mud" is used to maintain the drilled hole stability, lubricate and cool the downhole equipment and carry the drill cuttings to the surface. It is imperative that this fluid be allowed to enter and fill the drill pipe when tripping the pipe into the hole and it is most convenient to allow the fluid to drain from the pipe when tripping out of the hole.
Certain drilling assemblies possess a positive displacement feature, which will not permit fluid to pass through the motor unless the motor is turning. Therefore, a by-pass or dump valve is positioned just above the motor assembly for the purpose of filling and/or draining the drill pipe.
In the prior art, such valves have employed a fluid velocity actuated piston. The piston action either opens or closes ports separating the inside of the tool from the annulus surrounding the tool.
When there is no drilling fluid circulation (such as during trips, surveying, etc.), a spring holds the piston, or throat, in an "up" or open port position, thus, allowing fluid entry and exit through the ports of the valve body.
Such piston type valves have the disadvantage that the drilling fluid cannot be circulated because the port is automatically closed by the drilling fluid. The valve piston is actuated by the velocity of the drilling fluid when the rig pump is started. The piston is forced down, closing off the ports, and thereafter, directing all fluid into the motor. When pumping is stopped, the spring returns the piston to the "up" or open port position. The capability to circulate the fluid without driving the motor is particularly desirable when it is necessary to discharge gas buildups from the drilling fluid.
Another type of valve which has seen use is ball-actuated. In such a device, the port is blocked by an inner sleeve retained by a sheer pin. A ball may then be dropped down and forced against the sleeve by the drilling fluid causing the sleeve to break the sheer pin and move into a position where the port is open. This type of valve works admirably to open the port for circulation, for example, where it is desired to permit escape of gases from the fluid. However, it requires that the string be filled from the inside, rather than employing a port as with the spring-biased piston valve. Furthermore, once the pin is sheered, the string must be removed from the hole.